Dipole moment A measure

The temperature in degrees Celsius at which the dielectric constant and dipole moment were measured is shown in this table in parentheses after the value. In some cases, the dipole moment was determined with the substance dissolved in a solvent, and the solvent used is also shown in parentheses after the temperature. 

Figure 4 shows the measured angle of 105° between the hydrogens and the direction of the dipole moment. The measured dipole moment of water is 1.844 debye (a debye unit is 3.336 x 10 ° C m). The dipole moment of water is responsible for its distinctive properties in the Hquid state. The O—H bond length within the H2O molecule is 0.96 x 10 ° m. Dipole—dipole interaction between two water molecules forms a hydrogen bond, which is electrostatic in nature. The lower part of Figure 4 (not to the same scale) shows the measured H-bond distance of 2.76 x 10 ° m or 0.276 nm. 

The extent to which molecules tend to orient themselves in an electrical field is a measure of their dipole moment. A polar molecule such as HF has a dipole moment a nonpolar molecule such as H2 or F2 has a dipole moment of zero. 

Another important linear parameter is the excitation anisotropy function, which is used to determine the spectral positions of the optical transitions and the relative orientation of the transition dipole moments. These measurements can be provided in most commercially available spectrofluorometers and require the use of viscous solvents and low concentrations (cM 1 pM) to avoid depolarization of the fluorescence due to molecular reorientations and reabsorption. The anisotropy value for a given excitation wavelength 1 can be calculated as... 

Physical properties of the solvent are used to describe polarity scales. These include both bulk properties, such as dielectric constant (relative permittivity), refractive index, latent heat of fusion, and vaporization, and molecular properties, such as dipole moment. A second set of polarity assessments has used measures of the chemical interactions between solvents and convenient reference solutes (see table 3.2). Polarity is a subjective phenomenon. (To a synthetic organic chemist, dichloromethane may be a polar solvent, whereas to an inorganic chemist, who is used to water, liquid ammonia, and concentrated sulfuric acid, dichloromethane has low polarity.)... 

A second use of microwave spectroscopy is the measurement of dipole moments. These are obtained by measuring the frequency shifts of lines in the applied electric field of a Stark-modulated spectrometer. This method of dipole-moment determination is superior to the older method of measuring dielectric constants. For example, impurities in the sample will not affect the dipole moment as measured by microwave spectroscopy. The dipole moment of a substance present to the extent of a few percent can be accurately measured if its microwave spectral lines can be assigned. The components of d can be determined, thus giving its orientation in the molecule, in addition to its magnitude. 

Su, Borho and Xu have applied rotational spectroscopic and high-level ab initio studies to the 1 1 chiral molecular adduct of propylene oxide dimer [117], Six homochiral and six heterochiral conformers were predicted to be the most stable configurations where each monomer acts as a proton acceptor and a donor simultaneously, forming two 6- or 5-membered intermolecular hydrogen-bonded rings. Rotational spectra of six, that is, three homochiral and heterochiral conformer pairs, out of the eight conformers that were predicted to have sufficiently large permanent electric dipole moments were measured and analyzed. 

Dipoles, in addition to having a certain size, also exist in specific directions. For example, the dipole between H and F points toward F since F has a larger electronegativity and draws the electron density toward itself. So the dipole has both a size and a direction. Measurements that possess both a size and a direction are known as vectors. The magnitude of the dipole is usually expressed as the dipole moment. A dipole moment is defined as the product of the charge, Q, times the distance between the charges, r. 

From (B + C)/2 and known monomer geometries, the distance between the monomers can be calculated, and by inference, the hydrogen bond length. In addition, the electric dipole moment a-component can be measured from line-broadening... 

Not all molecular vibrations absorb infrared radiation. To understand which ones do and which do not, we need to consider how an electromagnetic field interacts with a molecular bond. The key to this interaction lies with the polarity of the bond, measured as its dipole moment. A bond with a dipole moment can be visualized as a positive charge and a negative charge separated by a spring. If this bond is placed in an electric field (Figure 12-3), it is either stretched or compressed, depending on the direction of the field. 

The experimental value of the total dipole moment as measured in the gas phase is found to be 0.550 0.040 D <1953JGS1622>. The p-d polarization also contributes to the total dipole moment value. Thus, depending on the method employed, a large variation is expected for thiophene (Table 9). 

Equation (16.3) is the simplest example of a direct electrostatic potential for two neutral molecules here, the dipole momentemergesas animportantphysicalproperty. Dipole moments are measures of the net separation of clrarge witlrin a molecule. For a spherically symmetric neutral clrarge distribution (e.g., air atom of argon), p. is zero. For a molecule in wlriclr clrarge I lq l is separated from charge — q by distance 1, tire dipole moment is p = q l. Hence p. 

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